A bipolar junction transistor (BJT) comprises three adjacent doped semiconductor regions or layers having an NPN or PNP doping configuration. A middle region forms a base and two end regions form an emitter and a collector. Typically, the emitter has a higher dopant concentration than the base and the collector, and the base has a higher dopant concentration than the collector. Generally, the BJT can be operated as an amplifier (for example, to amplify an input signal supplied between the base and the emitter, with the output signal appearing across the emitter/collector) or as a switch (for example, an input signal applied across the base/emitter switches the emitter/collector circuit to an opened or a closed (i.e., short-circuited) state). In operation, the emitter/base pn junction is forward biased and the collector/base pn junction is reverse biased. According to convention, all BJT currents are assumed to be positive when flowing into the base, collector and emitter regions of the BJT.
The breakdown characteristics of a BJT are determined by its physical parameters, including doping levels and region dimensions. The reverse-biased base-collector junction is susceptible to avalanche breakdown, during which a carrier of the reverse saturation current falls down a potential barrier at the reverse-biased junction and acquires energy from the applied potential across the junction. For example, in an isolated pn junction, if the reverse bias voltage creates a sufficiently large electric field in the transition region, an electron of the reverse saturation current entering from the p-type material may acquire sufficient kinetic energy to cause an ionizing collision with the lattice, creating an electron-hole pair. The original electron and the secondary electron are swept into the n-material by the electric field, and the secondary hole is swept to the p-side. The generated secondary carriers (both the electron and the hole) may acquire sufficient energy from the applied field, collide with another crystal ion, and create still another electron-hole pair. Each new carrier may, in turn, produce additional carriers through collision and disruption of existing bonds. The cumulative process is referred to as avalanche multiplication, where an initial carrier can create a large number of new carriers. Increasing the reverse bias voltage increases the energy imparted to the carrier, in turn increasing the rate at which additional carriers are generated by collisions, i.e., increasing the avalanche multiplication factor.
In a common base configuration with the emitter open, the breakdown due to impact ionization produces a breakdown voltage (BVCBO) (referred to as the common base breakdown voltage with the emitter open) above which the collector current increases without bound and is essentially limited by the external circuit resistance, i.e., the circuit in which the transistor is operative. This breakdown phenomenon is essentially identical to the breakdown of an isolated pn junction as described above. With the emitter current set at zero (because the emitter is open) transistor action plays no role in this breakdown mechanism. The value BVCBO thus represents an upper operational limit for the collector-base reverse-bias voltage of a transistor. Since the sum of the collector-base voltage and the base-emitter voltage equals the collector-emitter voltage and the base-emitter voltage is constant, BVCBO can also be regarded as an upper limit on the collector-emitter voltage.
For the common emitter configuration with the base open, avalanche breakdown occurs at a voltage BVCEO (referred to as the common emitter breakdown voltage with the base open), where BVCEO<BVCBO due to the influence of transistor action on the avalanche multiplication process.
In the BJT, during avalanche breakdown carriers generated by impact ionization in the collector-base depletion region are injected into the base or swept into the collector. The carriers injected into the base create a base current that in turn leads to an increase in the emitter current that in turn increases the carriers injected from the emitter. A current runaway situation may eventually develop as the avalanche mechanism snowballs. The current of injected carriers is multiplied by the avalanche multiplication factor at operating voltages well below BVCEO, and as BVCEO is approached breakdown of the base-collector junction is observed, i.e., the collector current increases without bound.
For example, in an NPN transistor, if the reverse bias voltage is sufficiently large, electrons collected in the collector may collide with a crystal ion in the collector depletion region, imparting sufficient energy to disrupt a crystal bond and create an electron-hole pair by impact ionization. The initial and secondary electrons are swept into the collector, while the secondary hole is swept into the base by the junction field. The generated carriers may acquire sufficient energy from the applied field, collide with another crystal ion, and create still another electron-hole pair. Each new carrier may, in turn, produce additional electron-hole pairs through collision and disruption of existing bonds.
To maintain charge neutrality in the NPN base, as holes flow into the base from the collector, additional electrons must be supplied to the base by the emitter. But the increase in electron injection causes an increase in the injection of electrons from the base into the collector that leads to the generation of additional secondary carriers at the base-collector depletion region. Thus transistor action contributes to the avalanche condition.
In a case where the emitter-base junction is slightly reverse-biased by imposing a small negative base current, the transistor can be operated above BVCEO as the negative base current minimizes carrier injection from the emitter into the base, minimizing the effect of transistor action on the avalanche condition and permitting operation of the transistor at a voltage above BVCEO (but not above BVCBO). Due to the negative base current, the emitter junction initially does not inject electrons into the base region. But, as the multiplication process increases hole injection into the base region to a point where the additional flow of holes into the base exceeds the negative base current, the holes flow into the emitter region. When hole injection into the emitter junction has increased sufficiently to cause significant electron injection into the base region, transistor action again begins to influence the avalanche process. An identical operating condition arises for a PNP bipolar junction transistor, with the references to holes and electrons reversed.
If the base current is positive or zero and the transistor is operated with a collector-base voltage above BVCEO, the avalanche process aided by transistor action causes the transistor to break down. However, if the base terminal can sink all the negative base current created during the avalanche multiplication process, the avalanche process does not cause a breakdown condition, unless the collector-base voltage is above BVCBO. Since in a typical bipolar junction transistor, BVCEO is much less than BVCBO, the transistor can be operated in the region between BVCEO and BVCBO if the base can sink the negative base current. In some transistor applications it may be necessary to compensate this negative base current to avoid errors in the transistor's output voltage.
In a typical bipolar junction transistor, the collector breakdown voltage (BVCEO) and the collector carrier transit time depend upon the thickness and doping concentration of the base and collector region. Lighter doping and a wider collector region increase the breakdown voltage and collector transit time. A narrower base decreases transit time (permitting operation at higher frequencies) but a higher collector-emitter voltage can destroy the narrow base. Ideally, transistors having both a high breakdown voltage and high speed performance flow transit time) are desired. Optimizing a bipolar junction transistor relative to these two countervailing effects necessarily results in a tradeoff between breakdown voltage and transit time (or speed, which is directly related to the maximum operational frequency for the transistor). A typical integrated circuit includes transistors that are optimized for high voltage operation and also transistors that are optimized for high speed performance, rather than attempting to produce a single transistor structure that is optimized for both.